Optical properties of heat-treated polyparaphenylene

The optical properties of heat-treated polyparaphenylene (PPP) were investigated by means of Raman and photoluminescence (PL) spectroscopy. Special attention is given to PPP heat-treated to temperatures (T-HT) near the carbonizing temperature region (T-HT approximate to 700 degrees C) since polymer-based carbonaceous compounds with low-T-HT (<1000 degrees C) have been found to exhibit electrochemical properties that strongly contrast both the as-prepared polymer and fully carbonized samples, The Raman spectra show that for T-HT in the range 650-725 degrees C, several Raman bands near 1300 cm(-1) can be correlated with both ground-state benzenoid and excited-state quinoid PPP A(g) modes. An increase in quinoid character is observed with increasing T-HT which is consistent with the theoretically predicted stabilization of the quinoid form in the presence of a high density of defects. The smaller energy bandgap for pi - pi* transitions in the quinoid conformation relative to that for the benzenoid form allows for a resonance condition to be present for laser excitation wavelengths (lambda(exc)) near the visible (similar to 1-2 eV). We also report a small dispersion effect in the observed quinoid breathing mode band which can be compared to dispersion effects previously reported for the case of trans-PA. The decrease in bandgap for the defect-induced quinoid form is also evidenced in the PL spectra of samples heat-treated up to 650 degrees C, which show vibronic structure in the blue-green emission data in the energy range 2.4-3.0 eV, with well-resolved peaks separated by quinoid phonon energies of 0.165 eV, Franck-Condon analysis shows an increase in the Huang-Rhys parameter (S) with increasing T-HT which can be related to changes in the electron-phonon coupling of valence and conduction band states.